1. Field of the Invention
The invention relates generally to vacuum arc remelting and more specifically to an apparatus and method to improve ingot quality by controlling the solidification of metal alloy ingots.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Vacuum arc remelting (VAR) is a process used widely throughout the specialty metals industry for the production of high quality ingots of reactive metal alloys, including, but not limited to, nickel-base superalloys, titanium alloys, zirconium alloys, and uranium alloys. In the VAR process, a cylindrically shaped, alloy electrode (1) is loaded into the water-cooled, copper crucible of a VAR furnace (FIG. 4). The furnace is then evacuated and a direct current electrical arc is struck between the electrode (cathode) and some starting material (e.g. metal chips) at the bottom of the crucible (anode). The arc heats both the starting material and the electrode tip (2), eventually melting both. As the electrode tip is melted away, molten metal drips off forming an ingot in the copper crucible. As the ingot grows, a molten metal, bowl-shaped pool (3) is located at the very top of the solidified ingot due to heat from the electrical arc and dripping liquid metal from the electrode. Because the crucible diameter is larger than the electrode diameter, the electrode must be driven downward toward the ingot pool in such a fashion so that the mean distance between the electrode tip and pool surface remains constant. The speed at which the electrode is driven down is called the electrode feed rate or drive speed. The mean distance between the electrode tip (2) and the ingot pool surface (4) is called the electrode gap.
Solidification control involves simultaneously controlling the electrode feed rate and the melting current with the ultimate goal of directly controlling the shape of the ingot pool. Control of the pool shape is very critical for the success of the VAR process because pool shape fluctuations are known to be related to the formation of material defects during solidification. Current state-of-the-art VAR controllers aim at indirectly controlling ingot pool shape in an open loop fashion by establishing controller set-points for the electrode gap and rate of electrode melting.
Direct control of the ingot pool shape requires a mathematical model that directly relates the process inputs to pool shape so that a pool shape set-point may be entered into the controller. Mathematical models of the VAR ingot solidification process have been developed and some are available commercially. Such a model usually consists of a set of differential equations describing heat conduction, fluid flow, electromagnetic phenomena and mass conservation in the ingot, combined with a specification of the inputs, material properties and ingot boundary conditions required to solve the differential equations. These models are very complex and require numerical methods for their solution. Examples include finite element, finite volume or finite difference methods. The models are non-linear and of very high order. As such, they are not useful for practical control systems design purposes, though very fast 2D ingot models may be useful for control feedback.
The prior art VAR control, which is currently widely used, controls electrode melting rate and electrode gap by using current and electrode drive speed. Typically, electrode gap is controlled indirectly by forcing the process to meet a drip-short frequency set-point reference or a voltage set-point reference. Melt rate control is established using electrode weight as a feedback variable to adjust melting current. The underlying assumption for this method of control is that holding the drip-short frequency (or voltage) and melt rate at their reference set-points produces a constant steady-state ingot pool shape which, in turn, ensures that favorable conditions in the ingot solidification zone are maintained, theoretically producing material free of solidification defects.
The prior art's method of VAR control has serious drawbacks. First, there are common process upsets (called MRE's or melt related events) that cause serious disturbances in melt rate. The only way these can be controlled is with large current responses. However, studies have demonstrated that large current variations affect solidification in the ingot as much as do large variations in melt rate. So controlling at a constant melt rate by allowing current variations has no advantage over holding the current constant and allowing melt rate variations. Neither method provides an effective means of controlling the ingot pool depth through normal process upsets, a problem directly addressed by the ingot pool shape controller of the present invention. A second drawback of the prior art VAR process control is that it is an open loop system with respect to pool depth or shape, the latter being the very process variable that really needs to be controlled. Thus, the currently used VAR process control does not respond to process upsets that affect pool shape but not melt rate. A method of closed-loop feedback control of the pool depth/shape, like the method of the present invention, enables one to control the pool shape directly and in response to process upsets that directly affect solidification. Additionally, the method of the present invention allows one to control the pool shape dynamically so that process control can be optimized during non-steady state operation at the beginning and end of the process.